PROBLEMS FROM DEVELOPMENT 59 



standing example of the increasing mineralization of water 

 with use. This problem of salinity increase is not solved by 

 limiting the application of irrigation water to the point of no 

 return to the ground-water reservoir, even if practical means 

 could be developed to provide only the quantities of water 

 required by the growing crops. All water applied for irriga- 

 tion carries some mineral matter. If the water is all returned 

 to the atmosphere by evapotranspiration, these salts are left 

 to accumulate in the soil. Effective irrigation practices must 

 include the removal of these salt residues from the soil by the 

 application of water in excess of the requirements of plants. 



W ellton-Mohawk Area of the Gila River Basin, Ariz. ie 

 The Wellton-Mohawk area is a ground-water reservoir 

 traversed by the Gila River some 20 to 50 miles above the 

 point where it flows into the Colorado River. The pump- 

 age from wells for irrigation increased from 35,000 acre- 

 feet in 1945 to 45,000 in 1949, of which perhaps one-fourth 

 returned to the ground-water reservoir by downward per- 

 colation. The rate of pumping has exceeded the replenish- 

 ment from all sources for a long time, and for 20 years the 

 water table under the irrigated areas has been lowered at an 

 average rate of about a foot a year. 



The mineral salts in the Wellton-Mohawk ground water 

 have always been high by national standards. Water from 

 wells near the margin of the district, probably derived from 

 local precipitation, commonly carries about 2,000 parts per 

 million of dissolved solids. The water pumped by irriga- 

 tion wells, derived chiefly by seepage from the Gila River 

 and by westward percolation in the alluvial materials, is 

 considerably more mineralized, and the concentration is 

 increasing. Twenty years ago, these waters carried as much 

 as 7,000 parts per million of dissolved matter, compared 



16 Reference: Babcock, H. M., S. C. Brown, and J. D. Hem, "Geology and 

 Ground-water Resources of the Wellton-Mohawk Area, 

 Yuma County, Arizona," U.S. Geol. Survey, Mimeo. rept., 

 1947, 22 pp. 



